Scissor-type jacks are used to apply force to lift many objects including motor vehicles. Many auto manufacturers include scissor type jacks as standard equipment for emergency tire changes. The compact size and ease of use make scissor-type jacks a popular choice for this type of application.
Generally, prior art scissor jacks include one pair of upper channel members and one pair of lower channel members, a base plate, a load bearing plate, pivot pins and a screw bar. The top ends of the upper channel members are pivotally mounted to a load bearing plate. The bottom ends of the lower channel members are pivotally mounted to a base plate. Each upper channel member is pivotally engaged to a lower channel member using a cylindrical pivot pin. These pins are herein referred to as trunnions. The trunnions have a centrally located through hole disposed perpendicular to their longitudinal axis.
A common scissor type jack employs one trunnion in which the through hole is threaded, and one trunnion with a plain through hole. A screw bar that is threaded on one end is placed through the plain through hole in one trunnion and is threadably engaged in the threaded trunnion. The trunnions are typically inserted through the channel members by way of circular holes formed in each of the opposed planar sides of each channel member. These holes are limited in size by the size of the channel members, as too large a hole would adversely weaken the channel members. The hole size in the channel limits the diameter of the cylindrical trunnion that can be inserted into the hole. The diameter of the cylindrical trunnion limits the diameter of the through hole centrally located in the trunnion, and also limits the length of engagement of the internal threads that are disposed in the hole of the threaded trunnion. In turn, the diameter of the through hole located in the threaded trunnion limits the diameter of the screw bar.
All of these limitations dictate the load capacity of the jack. Destructive testing has shown that a typical failure location for a scissor jack occurs at the internal threads of the threaded trunnion. Therefore, it would be desirable to increase the thread area of the threaded trunnion, thereby increasing the length of engagement between the threaded trunnion and the screw bar, which would increase the load capacity of the jack. The easiest way to accomplish this is to increase the diameter of the cylindrical trunnions, thus allowing a longer through hole and therefore more threads. This however would require an increase in the size of the channel members to accommodate the larger hole required for the larger diameter cylindrical trunnions. Such a size increases would demand more storage space be allotted in the trunk of a vehicle for the larger jack, and the corresponding increase in weight would be undesirable by making the jack more difficult to maneuver and operate.
An alternate method of increasing the length of thread engagement requires that a tube with internal threads be affixed to the cylindrical trunnion. This is disclosed in U.S. Pat. No. 5,449,149. This method makes the jack more difficult to manufacture. Using a finer thread pitch on the screw bar and in the threaded trunnion would also increase the strength of the threaded connection, but would require more revolutions of the screw bar to achieve the same vertical displacement. Therefore, it would be desirable to increase the load carrying capacity of the jack without a dramatic increase in the weight of the jack, and without making the use and manufacture of the jack more complex.
The cylindrical trunnions are secured in place by a variety of methods including spring clips, bolts, nuts and swaging the ends of the trunnion. These methods also strengthen the channel member by creating a flange surface to prevent the channel sides from spreading under a load. However, these methods all add parts and/or production steps. Therefore, it would also be desirable to have a method to secure the trunnions with a flange formed integrally to the trunnion.
In light of the deficiencies in prior art, the applicants' invention is herein submitted.